A cable modem termination system (CMTS) is equipment typically found in a cable company's headend (hubsite) and is used to provide high speed data services, e.g., cable internet or Voice over IP, to cable subscribers. A CMTS often functions as a router with Ethernet interfaces (connections) on one side and coax RF interfaces on the other side. The RF/coax interfaces may carry RF signals to and from the subscriber's cable modem.
CMTSs typically carry only IP traffic. Traffic destined for the cable modem from the Internet, often designated as downstream traffic, is carried in IP packets encapsulated in Moving Picture Experts Group (MPEG) transport stream packets. The MPEG packets are carried on data streams that are typically modulated onto a TV channel using Quadrature Amplitude Modulation (QAM). Upstream data (data from cable modems to the headend or Internet) is carried in Ethernet frames modulated with QPSK, 16-QAM, 32-QAM, 64-QAM, or S-CDMA. Transmission is often through the sub-band portion of the cable TV spectrum (also known as the “T” channels), which is a lower part of the frequency spectrum than the downstream signal.
In order to provide high speed data services, a cable company typically connects its headend to the Internet via very high capacity data links to a network service provider. On the subscriber side of the headend, the CMTS enables the communication with subscribers' cable modems. Different CMTSs are capable of serving different cable modem population size, ranging from 4,000 cable modems to 150,000 or more, depending in part on traffic. A given headend may have between half a dozen to a dozen or more CMTSs to service the cable modem population served by that headend or hybrid fiber coax (HFC) hub. CMTSs may have both Ethernet interfaces as well as RF interfaces. In this way, traffic that is coming from the Internet can be routed through the Ethernet interface, through the CMTS and then onto the RF interfaces that are connected to the cable company's HFC hub. The traffic typically winds its way through the HFC to end up at the cable modem in the subscriber's home. Traffic going from a subscriber's home systems go through the cable modem and out to the Internet in the opposite direction.
Cable subscribers are typically assigned to a specific CMTS, in which each subscriber is provided grades of data services. It is therefore important that the cable provider engineer the CMTSs so that subscribers experience the expected quality of service.